is a versatile and widely used molecular simulation program with broad application to many-particle systems

has been developed with a primary focus on the study of molecules of biological interest, including peptides, proteins, prosthetic groups, small molecule ligands, nucleic acids, lipids, and carbohydrates, as they occur in solution, crystals, and membrane environments

can be utilized with various energy functions and models, from mixed quantum mechanical-molecular mechanical force fields, to all-atom classical potentials with explicit solvent and various boundary conditions, to implicit solvent and membrane models

has been ported to numerous platforms in both serial and parallel architectures

Note that CHARMM comes as a stable version (with the letter "b" in the version information, eg c34b1) and the development version with the letter "a" (eg c35a2). Only Charmm developers have access to the development version. There is also a special forum available only to developers which sometimes has much more up-to-date information than the publicly accessible forum. In case of severe problems it can be a good idea to contact your next Charmm guru directly.